Original plan was to install and play each expansion one by one, in order, but since 2008 I rarely had time to play games, so after 3 years, in November 2011 I decided to jump to Apartment Life, then in mid-2012 I downloaded all expansions and stuff packs to do an one-day test for each one, I do not have time to play more.
EP1 University (played since 2008): useless expansion, the single good thing is that it is adding the cheat sethighestallowedlevel that allow you to build up to 16 floors. University do not change the format of game saves. Lots saved with University expansion pack, can be open and played with base game only or with any other expansions but without University, any University object is deleted rather than crashing the game. University lots in base game will have crazy prices, even negative prices, like $-987,123,456.00, opening lots and saving them again will fix the price.
EP2 Nightlife (played since 2009): I love that it added cars and disco/party features.
EP3 Open For Business (played June 2012): was this really necessary? It add elevators and modern windows, but these were re-added by Apartment Life. Open for Business and University are the ONLY expansion packs that add the cheat sethighestallowedlevel.
EP4 Pets (played June 2012): good expansion for animal lovers like me! Otherwise, having pets in family is annoying.
EP5 Seasons (played February 2012): I think that weather in The Sims 2 was necessary, as well as the gardening, too bad that they did not made the weather properly.
EP6 Bon Voyage (played July 2012): useless expansion, I do not enjoy sending sims in vacation.
EP7 Free Time (played July 2012): useless expansion.
EP8 Apartment Life (played since November 2011): definitely the best expansion! Not only that it allows up to 4 families on same lot, but it includes elevators, spiral stairs, ceilings (helpful for making beautiful indoor photos) and advanced decoration features!
Additional info: All expansions since Nightlife onward add more lot sizes, from 3×1 and 2×2 to 6×5, remove the restriction of making non-rectangular pools, and add the cheat to change roof slope angle. These are must-have features that shall be included since base game.
Stuff Packs: In July 2012 I also tested all stuff packs. Mansion and Garden is the best, but none of them is really useful, so all my houses that I designed in The Sims 2 do not require stuff packs. Stuff packs do not improve the game, these just add more Buy mode items as well as more floors and walls covers.